Experts who tried and failed to lock themselves into a sports bag identical to that in which MI6 officer Gareth Williams was found believed it likely a third party was involved, his inquest heard, as it emerged he browsed self-bondage websites and videoed himself naked in boots.

Peter Faulding, who tried 300 times, said he believed Williams was either dead or unconscious when placed in the holdall.

The former Parachute Regiment reservist, who specialises in rescuing people from confined places, described theories that Williams got into the holdall by himself as "unbelievable scenarios".

Even the great escapologist Harry Houdini would have struggled to squeeze into the 81cm x 48cm bag, Westminster coroner's court heard.

His conclusion was that the codes and ciphers expert was either placed in the bag unconscious, or was dead before he was put in the bag.

The naked and decomposing body of Williams was found in a red North Face holdall in the bath in his top floor flat in Pimlico, central London, on 23 August 2010.

The inquest was shown a video of Faulding's attempts, as he explained it would have been extremely hot in the bag, and Williams would have been able to survive for 30 minutes at most.

A second expert who, with an assistant, tried and failed 100 times to padlock the bag from the inside, refused to rule out the possibility that Williams, who was extremely fit and an accomplished mountaineer and fell runner, had locked himself in.

"I would not like to say it could not be done: there are people around who can do amazing things and Mr Williams may well have been one of those persons," said William MacKay

Mountaineering experience would have strengthened Williams's fingers. But Mackay said: "It was a very painful thing to do it. You tend to move the zip with your finger nails, straggling about … it was very frustrating, fiddly, you just can't get the thing together."

He said it was possible for someone locked inside the bag who wanted to get out to poke a small hole through the zip with their fingers.

This would give them more oxygen and allow them to unlock the padlock as long as they had the key.

"There was an option of saving oneself," he said. The holdall was not airtight, and as long as the person did not panic they would be able to breathe for some time.

"If the person was in the bag willingly, then I am tending more towards the heat side being a problem than the oxygen side," he added.

The inquest has heard that keys which fitted the padlock were found under Williams's right buttock.

Williams, who had a £20,000 collection of women's designer clothes and shoes, appeared to have videoed himself from behind, bending down while wearing nothing but black leather boots, the hearing was told.

Describing the footage, found on a mobile phone, DC Robert Burrows said he appears to reach down and touch his boots before he "wiggles and gyrates his posture", with his buttocks pointed towards the camera.

Examination of Williams' computer and phones showed pictures of drag queens, and that he browsed self-bondage websites.

He spent 50% of his time online looking at couture women's fashion, but had also looked at fetish websites.

One image viewed showed "a model enclosed in plastic and using a vacuum cleaner to suck the air out", said Burrows.

Pages from 2008 showed he may have looked at sites relating to "hogtie", a bondage method of tying the limbs together, and he once watched a YouTube video after typing in "dress bondage training".

But the browsing was a tiny percentage and he had not, as has previously been reported, visited claustrophilia websites, the inquest heard.

It also emerged that the US conference Williams attended for work and had recently returned from before his death was a "hacking convention" which was attended by ethical and criminal hackers.

The hearing was adjourned until Monday.

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